The invention concerns an optical method of measuring at least one dimension of an object. It also concerns a method of manufacturing an object, notably a machining method, employing the measuring method. It further concerns an object produced by the method of manufacture referred to above. It finally concerns a timepiece movement or a timepiece, notably a wristwatch, including such an object.
The development of machine tools aims to improve accuracy, via the reduction of machining errors. As one of a number of examples there may be cited mechanical timepieces, where the aim is still and always to improve the accuracy of the manufacture of the components in order to improve the performance of the movements and the throughput of the assembly production line. Other examples concern the automotive, medical, space, aeronautic and electronics industries.
This search for improvement is made difficult by the working conditions of machine tools. This is particularly true for bar turning machines whether of the cam type or numerically controlled, but also for all other machines using a process of removing material, in particular by the formation of swarf. It proves difficult to improve further the intrinsic accuracy of these machines by the usual means, such as optimizing the architecture or the accuracy of the guides. In fact, residual machining errors, like errors caused by thermal distortion, static distortion, non-repeatable positioning of mobile elements (devices supporting the tools) and wear of the tools, have reached a minimum threshold that it appears difficult to go beyond. These residual errors remain too large for some applications, such as watchmaking, however.
Machine tools are ill-equipped for improving performance by installing in-situ sensors, as the latter are severely compromised by the severe environment brought about the presence of the cutting liquid and swarf. A number of ways have previously been examined to improve repeatability performance using sensors that measure the workpieces, the position of the tools or certain mobile elements of the machine during the production process, and afterwards exploiting these measurements to control or to correct the machine in real time.
Machine tools, such as bar turning machines (but also lathes, transfer machines, etc) generally include one or more devices for correcting machining errors noted during the production of the workpieces. These devices are either numerical tool correctors in the case of a numerically controlled machine tool or micrometer screw devices in the case of cam-type machines. Corrections are generally made manually by the operative monitoring the machine on the production line.
There exist various systems for measuring the mobile machine elements, such as position sensors that are mounted on the spindles of the machines, for example, such as LVDT inductive sensors or optical rules, for example. These sensors do not measure the dimensions of the manufactured workpiece, however.
There are also solutions that directly measure the workpiece, such as laser barriers, for example, or other optical gauge systems, or systems using some other physical principle. In principle, this type of measurement works by triggering contact. The measurement is then “read” on the axis tracking systems of the machine tool. This approach of measurement via axis tracking sensors does not make it possible to achieve the required performance. Moreover, in the case of small workpieces, most of these solutions are highly “invasive” and therefore particularly difficult to implement.
To summarize, because of their intrinsic shortcomings, none of these measuring solutions makes it possible to achieve the resolution and the repeatability necessary for accurately measuring the sizes or the dimensions (diameters, lengths) of an object, and in particular of a timepiece component of the millimeter scale revolution object type.
The document JP2008102040 describes a measuring device for measuring the diameter and the concentricity of the various sections of a cylindrical workpiece by rotating the workpiece. No information is given on positioning the workpiece in the optical system.
The document FR2646904 describes a method of measuring the diameter of a cylindrical object along the object by a vertical displacement and a displacement in rotation. An illumination system and a detector of the one-dimensional type are used. There is no particular reference to positioning the object in the system.
The document US2002041381 describes a device for measuring the diameter and the concentricity of a cylindrical object in a telecentric type optical system with a combination of two sensors, one of the one-dimensional type, the other of the two-dimensional type. Positioning the object in the system is not a topic in this document.
The document US2012194673 describes a measuring system of the reflection microscope type with a table that can be moved along an optical axis in order to acquire a series of images at different working distances. This series of images makes it possible to determine the focal points of the various levels of the workpiece; an image is then taken at each position z of interest for measuring the dimensions of interest.